Design Criteria

Standard girder designs must meet the following requirements:
  • Specify fit condition in the plans when necessary as recommended in Article 6.7.2, and specify steel dead load fit (SDLF) where possible.
  • Diaphragm and cross-frame designs must meet the following requirements:
    • The maximum spacing is 30 ft. if all limit states requirements are met.
    • Provide diaphragms/cross-frames at all end bearings. At least two interior bearings at a bent must have a diaphragm/cross-frame intersecting them.
    • Set interior diaphragms/cross-frames parallel to bents or abutments for skews up to 20°. Set interior diaphragms/cross-frames perpendicular to girders for skews beyond 20°.
    • Check the limiting slenderness ratio of cross-frame members using criteria provided in Articles 6.8.4 and 6.9.3.
  • Lean-on bracing design, as described in
    Cross-Frame and Diaphragm Behavior for Steel Bridges with Skewed Supports, Helwig and Wang, Research Report 1772-1, 2003,
     is permissible. For structures utilizing lean-on bracing systems, detail the assumed construction sequence in the plans.
Girder designs must meet the following requirements:
  • Use composite design and place shear connectors the full girder length.
  • Use short-term modular ratio equal to 8 and long-term modular ratio equal to 24.
  • Provide longitudinal slab reinforcement in accordance with Article 6.10.1.7.
  • Assume the composite slab is effective in negative bending regions for Deflection check, Fatigue and Fracture Limit State, and Service Limit State. When calculating stresses in structural steel for composite sections in negative bending for the Service II Limit state, only include the concrete deck in the section properties if tensile stress in the deck is less than 2fr
    per Article 6.10.4.2.1.
  • At flange splices, extend thicker flanges beyond the theoretical flange splice location by a length equal to the flange width but not more than 2 ft.
  • Include an assumed stay-in-place formwork weight of 15 psf in design.
  • Investigate and verify feasibility of a possible erection sequence during design and verify possible locations of shore towers and cranes. Consider traffic phasing with underlying roadways when considering locations of shore towers and cranes. Do not include detailed erection plans in plan set.
  • Specify continuous placement of bridge deck where possible, and staged placement only if required. Do not disallow continuous placement solely based on whether a continuous pour may be unfeasible for a contractor. If staged placement is specified, base girder design on the worst-case effect of staged and continuous placement. Base dead load deflection and camber on an analysis for staged placement if staged placement is the only placement option. If both staged and continuous placement are given as options, base dead load deflection and camber on continuous placement as long as there is no significant difference in final camber and deflection between the two methods. State in the plans which placement option is assumed for the dead load deflection and camber. Provide a staged placement diagram indicating the intended pour sequence in the design if staged placement is specified. In the plans, state that for continuous placement, the minimum rate of placing and finishing shall not be less than that specified in Item 422.
  • For stud connector designs, minimum longitudinal stud connector spacing is limited to 4
    d
    , where
    d
     is the stud connector diameter. Do not exceed a stud connector spacing of 24 in. regardless of girder depth.
  • For dapped girder ends, utilize Article D6.5.2 to avoid the use of additional stiffeners at dap bend points per Article 6.10.1.4.
  • In lieu of permanent bottom flange lateral bracing, increase bottom flange size if practical. If considering the use of bottom flange lateral bracing, contact TxDOT Bridge Division - Design Section for approval.
Provide bolted field splices as the primary method of field splicing in the plans. Include the weight of the splice plates in the steel weight for payment. Bolted field splices must meet the following requirements:
  • Use ASTM F3125 Grade A325 bolts. Use galvanized Grade A325 bolts for painted structures. Use Grade A490 bolts only if the connection cannot be designed with A325 bolts. Do not specify galvanized Grade A490 bolts for any structure.
  • Assume Class A surface conditions. Class B surface conditions may be used only when slip controls the number of required bolts. Always note the surface condition assumed for design in the plans.
  • Add at least 0.125 in., and preferably 0.25 in., to minimum edge distances shown in Table 6.13.2.6.6-1.
  • Do not extend and develop fill plates equal to or thicker than 0.25 in. Instead, reduce bolt shear strength with Equation 6.13.6.1.4-1.